Wireless communication systems are assuming ever-increasing importance for the transmission of data, which is to be understood in its largest sense as covering speech or other sounds and images, for example, as well as abstract digital signals.
Currently proposed standards for wireless communication systems include the 3GPP (3rd generation Partnership Project) and 3GPP2 standards, which use Code Division Multiple Access (‘CDMA’) and Frequency Division Duplex (‘FDD’) or Time Division Duplex (‘TDD’), the HIPERLAN and HIPERLAN2 local area network standards of the European Telecommunications Standards Institute (‘ETSI’), which use Time Division Duplex (‘TDD’) and the International Telecommunications Union (‘ITU’) IMT-2000 standards. The present invention is applicable to systems of these kinds and other wireless communication systems.
In order to improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, various techniques are used separately or in combination, including space-time diversity, where the same data is transmitted over different transmit and/or receive antenna elements, and frequency spreading, such as Orthogonal Frequency Division Multiplex (‘OFDM’) where the same data is spread over different channels distinguished by their sub-carrier frequency.
At the receiver, the detection of the symbols is performed utilising knowledge of the complex channel attenuation and phase shifts: the Channel State Information (‘CSI’). The Channel State Information is obtained at the receiver by measuring the value of pilot signals transmitted together with the data from the transmitter. The knowledge of the channel enables the received signals to be processed jointly according to the Maximum Ratio Combining technique, in which the received signal is multiplied by the Hermitian transpose of the estimated channel transfer matrix.
Two broad ways of managing the transmit diversity have been categorised as ‘closed loop’ and ‘open loop’. In closed loop signal transmission, information concerning the transmission channels is utilised at the transmitter to improve the communication. For example, the document Tdoc SMG2 UMTS-L1 318/98 presented to the ETSI UMTS Physical Layer Expert Group describes operation of a Transmit Adaptive Array (Tx AA) FDD scheme in which the dedicated channels are transmitted coherently with the same data and code at each transmit antenna, but with antenna-specific amplitude and phase weighting. The receiver uses pilots transmitted on the Common Channels to estimate separately the channels seen from each antenna. The receiver estimates the weights that should be applied at the transmitter to maximise the power received at the receiver, quantises the weights and feeds them back to the transmitter. The transmitter applies the respective quantised weights to the amplitudes and phases of the signals transmitted from each transmit antenna of the array. Alternatively, in TDD systems, the channel state information for weighting the signals applied to the downlink transmit antennas may be derived from the uplink signals, assuming that the channels are equivalent, without transmission of any specific channel or weighting information from the receiver to the transmitter.
Multi-Transmit-Multi-Receive (‘MTMR’) diversity schemes, where essentially the same signal is transmitted in space-time diversity over the different combinations of transmit and receive antenna elements, can provide significant gains in Signal-to-Noise Ratios (‘SNR’) and thus operate at low SNRs, enabling an increase in spectral efficiency via the use of high order modulations. Alternatively, in multi-stream wireless communication schemes, different signals can be transmitted between the transmit and receive antenna element arrays enabling high spectral efficiency. However, multi-stream schemes of this kind that have been proposed are viable only at high SNRs and require complex receivers (for a N-Transmit and M-Receive antenna configuration, M must be greater than or equal to N) in order to be able to extract the different transmitted signals at the receiver.
An example of an open-loop multi-stream single user scheme is the Bell Labs layered space-time (‘BLAST’) scheme described in an article by G. J. Foschini entitled “Layered Space-Time Architecture for Wireless Communication in a fading Environment When Using Multiple Antennas,” Bell Laboratories Technical Journal, Vol. 1, No. 2, Autumn, 1996, pp. 41-59.
A closed-loop alternative to the above scheme in which channel knowledge is used at the transmitter for multi-stream transmission is described in an article by Mansoor Ahmed, Joseph Pautler and Kamyar Rohani entitled “CDMA Receiver Performance for Multiple-Input Multiple-Output Antenna Systems,” Vehicular Technology Conference, Fall, Atlanta City, October 2001. A schematic diagram illustrating the principle of this communication system is shown in the accompanying FIG. 1.
Such schemes are limited by compromises between diversity gain and spectral efficiency and accordingly the range of operational SNRs is limited unless complexity is increased or high modulation constellations (for example greater than 64 QAM) are used. The present invention offers a substantial improvement in the compromise between diversity gain and spectral efficiency.